High-frequency amplifier



Sept. 22, 1953 D. H. SLOAN HIGH-FREQUENCY AMPLIFIER Filed April I4, 1951 5 Sheets-Sheet l Sept. 22, 1953 D. H. SLOAN HIGH-FREQUENCY AMPLIFIER 3 Sheets-Sheet 2 Filed April 14, 1951 INVENTUR.

A TTORNEYS Patented Sept. 22, 1953 HIGHFREQUENCY AIVIPLIFIER David H. Sloan, Berkeley, Calif., assignor to Research Corporation, New York, N. Y., a corporation of New York Application April 14, 1951, Serial No. 221,017

9 Claims. 1

This invention relates to vacuum tube structures and particularly to structures used in the type of vacuum tubes known as resnatrons." Such tubes are particularly adapted for the generation or amplification of electrical oscillations in the ultrahigh frequency and microwave ranges, and are described, for example, in the prior Patent No. 2,424,002, issued July 15, 1947 to Sloan and Marshall. From one point of view tubes of this character can be considered as comprising two or more cavity resonators which are electroncoupled. The cathode is positioned within one of the cavities and directs a beam of electrons across the cavity toward a portion of the opposite wall thereof which is electron-permeable; i. e., it is apertured so as to form a control grid. An electron-collecting anode is formed within a second cavity in such position as to receive that portion of the electron beam passing through the grid structure. Resnatrons may be either of the triode or the multi-electrode type; if the latter the cathode-grid cavity and the anode cavity are spaced apart, the intervening space usually, although not necessarily, non-resonant at operating frequencies. The wall of the anode cavity is apertured to admit the electron beam to the anode cavity and anode.

In past practice a second or accelerating grid structure has been placed in the general path of the electron beam to act as an accelerating electrode. Since the tubes are primarily designed to operate on very high power, with accelerating potentials of many thousand volts, the amount of energy lost as heat when electrons are intercepted by the accelerating grid structure is very material. Accordingly the attempt has been made to place crossbars of the accelerator grid in the areas shielded by the control-grid bars. Where the tubes are used as self-excited oscillators, however, the potentials are usually adjusted so that the transit time of the electrons between cathode and anode is an integral number of half-cycles. The relative voltages of the various electrodes are therefore changing very rapidly during the transit of the electrons. The gaps between the grld bars therefore tend to act as electron lenses and since the relative potentials are changing, the focal lengths of these electron lenses are also changing concurrently. Hence, although the grid bars may be placed so that a beam passing from the control grid through the accelerator grid is focused at the time of greatest electron density within the stream and most of the electrons miss the grid bars, as the relative potential changes a de-focusing effect occurs and a considerable number of the electrons do strike the bars. This causes a waste of energy which might be tolerated in devices utilizing lower power but in a device of the present character, where the power waste may run into losses of the order of many kilowatts, not only is the waste itself important but the problem of getting rid of the useless heat becomes one of major importance and may be the factor which ultimately limits the power output obtainable from the device.

It is well known that cavity resonators may be excited in a theoretically infinite number of modes and that several such modes may co-exist within such a device. In general it is required for satisfactory operation that a tube of the character described oscillate in only a single mode, the co-existence of any other mode resulting in a useless waste of power so that such second mode is termed parasitic." Early resnatrons were therefore all designed, where possible, so that they would tend to oscillate only at one of the lowest frequency modes of the cavities involved. In my co-pending application Serial No. 771,852, however, flied September 3, 1947, I have shown that it is possible to operate a tube of this character stably at higher modes. While the oscillating cavities may theoretically be of almost any shape, practical consideration leads to the use of the cavities having circular symmetry. The readiest method of causing such a cavity to oscillate at its principal mode is to concentrate the electron beam along the axis of symmetry, a potential loop within the cavity being formed at this point. My prior application Serial No. 771,- 852 discloses, however, that by taking certain precautions, there described, an annular electron beam may be used traversing the cavity one-half wavelength radially outward from the axis at a potential loop of the higher mode chosen for operation of the device. This makes possible the use of a cathode oi much larger area with the consequently much greater electron current and higher output from the tube. The present invention relates specifically to resnatron-type tubes wherein such annular cathodes are used.

While the invention could be used in connection with pentodes or even with tubes using a greater number of electrodes, experience has shown that thus far the most satisfactory method of operating a resnatron is as a tetrode. Accord ingly resnatrons oi the tetrode type will be used in the illustrative embodiments of the invention described herein.

Thebroad purpose of the present invention is to provide a resnatron structure, primarily for use at, very high powers. which substantially eliminates the loss of power through interception of electrons by the accelerating. electrode. Contributory to this broad purpose, among the objects of the invention are to provide a structure wherein a narrow annulus of electron flow is focused, either axially or radially, within an annuiar slot in the structure forming an accelerating electrode, no material deflection of the electrons constituting the beam occurring circumferentiaily of the annulus; to provide a structure of the character described wherein the direction of flow of the electrons constituting the beam is determined primarily by the formation of the cathode, to an extent such that changes of relative potential between the electrodes do not materially disturb the focus; to provide a general type of structure which may be used either in tubes wherein the flow of electrons in the beam is parallel to the axis (hereinafter referred to as "axial flow" tubes), or in resnatrons wherein the flow is of the radial type; and, concurrently, to provide a structure which may itself be used to suppress certain types of parasitic modes of operation within the tube which otherwise would be likely to occur.

In accordance with the present invention the fundamental resnatron structure which has already been described is retained. Both the oathode-grid cavity and the anode-accelerator cavity are, however, essentially annular, although one or the other (usually the grid-cathode cavity) may be coupled with and either feed from or feed into a circular cavity of the general type which has previously been disclosed. The cathode, located within the cathode-grid cavity, is itself annular in form. It may consist of many individual cathode e1ementsi. e., filamentsin par allel, or may be a unitary structure of the indirectly heated type, but preferably its emitting surface is concave, thus tending to produce a converging beam of electrons.

The anode-accelerator cavity is positioned with the electron-receiving surface of the anode in the path of the annular electron beam generated by the cathode. The beam enters the anode cavity through a substantially unobstructed slot in the cavity wall directly opposite the anode proper. It is usually preferable to form the slot between flanges which project toward either the anode, the cathode, or, preferably both, this arrangement permittlhg lower capacities between accelerating electrode and both the grid and the anode structure, permitting a somewhat higher impedance cavity to be formed. The opposed faces of the flanges are also preferably formed so as to conform generally to the electron paths defining the edges of the annular beam and are so spaced that the width of the slot is only suflicient to admit substantially all of the electrons forming the beam. The slot is, as has been stated, substantially unobstructed, in that it is not interrupted by accelerator-grid bars which are separad by angular distances of the same order of magnitude as the spacing of the control-grid bars (the latter, in a characteristic tube, being spaced at angles of somewhere between one and two degrees). However, it may be advisable to have a few widely separated transverse conductors bridging the slot and contributing very little to the electronic interaction property of the grid. Such conductors can serve two purposes; first, they may serve as mechanical supports, tying the structure together, and second, they may serve to break up possible undesired modes of oscillation which might otherwise'be set up within the cav ity. If the structure of the tube is such that only the mechanical purpose need be fulfilled, three or four such bridging conductors would normally be used, but if the electrical function is also required the number of bridging conductors might be as high as ten or twelve but probably not more.

In the detailed description which follows this will be more clearly explained, the said description being illustrated by the accompanying drawings wherein:

Fig. 1 is a diagrammatic axial section of a resnatron of the axial flow type which embodies the instant invention;

Fig. 2 is a transverse sectional view of the tube illustrated in Fig. 1, the plane of section being indicated by the line I-I of the first figure; and

Fig. 3 is a longitudinal section of a radial flow type of resnatron, the view also being partly in diagrammatic form.

As has been indicated all of the figures of the drawing are diagrammatic in nature, it being believed that the simplification thus achieved will lead to a better understanding of the actual invention. Most of the electrodes to be described are, in fact, composite structures built up of numerous parts, threaded, welded, or brazed togather. Detailed construction of similar structures is shown in the prior patent and application mentioned above and those to be described may be built up in like manner. Electrically, however. the electrodes are unitary and both the specification and the drawings can be much simpliiled by so considering them.

Fig. 1 illustrates a resnatron of the axial flow type which embodies the present invention. The mechanical framework of the tube comprises a cathode-end disc I and an anode disc 3, both provided with circular flanges, i and 1 respectively, which project toward each other from the discs and are separated by an insulating glass ring 9 sealed to the flanges and serving simultaneously as mechanical connection and electrical insulation between them. Axially the ring 8 must be of suflicient length to prevent flashover between the discs at operating potentials of several thousand volts-upon occasion voltages as high as one hundred thousand. The glass ring preferably bulges outwardly somewhat, as shown, to enable it better to withstand the atmospheric pressure, since the space enclosed by it is, of course, evacuated.

A cylindrical cathode-grid support column ll projectes inwardly from the disc I, coaxial therewith. This column, in the particular tube illustrated, is slightly more than one-half the diameter of the disc I. It terminates in a cathode flange I3 of the same internal diameter as the column and a grid support flange I5 which surrounds the cathode connector I3 and projects axially beyond it, leaving a groove ll between the outer periphery of the element II and the inner edge of the flange I5.

Extending outwardly from the disc i and coaxially therewith is a short cylindrical flange i9, surrounding a central opening in the disc. An insulating ring or gasket 2| presses against the end of the flange it. The ring 2| supports, in turn, an outer flange 23 which projects toward the anode disc from an annular foot 25 supporting a tubular cathode column 21, the latter extending through the central opening in the disc I toward the anode. The anode end of this column is terminated by a frusto-conical flange 29,

flaring out from the column and terminating in a flat surface 3| in substantially the same plane as the end of the cathode support element 13. The outer and larger diameter end of the flange 29 is sufliciently smaller than the inner diameter of the column II to leave a gap between them; the scale of the drawing of the speciflc tube shown may be gauged by the fact that in this particular tube this gap is approximately one thirty-second of an inch in width.

The gap mentioned is bridged by the cathode structure. In the present instance this consists of a large number of staple-shaped thoriated tungsten ribbon filaments 33. Three hundred and six of these filaments are used in the tube described but more or less could be employed in any other specific design. Each filament is bowed as shown to present a concave surface toward the anode of the tube. The use of the multiple filaments is here adopted from purely mechanical and thermal considerations; electrically the structure is unitary and can be considered as though it were a single cathode. A fully annular cathode could be used and indirectly heated, but in this case also the cathode would be formed as an annulus with a concavity as viewed from the anode.

Grids for controlling the electron flow in the tube are positioned closely adjacent the concave surface of the cathode structure. In the present instance the grid members comprise arcuate tungsten strips 35, mounted edge-on with respect to the cathode structure. These strips are formed and positioned with their centers of cur vature approximately coinciding with that of the cathode concavity. The edgewise-on tungsten strips are set in slots formed, respectively, in the edge of the grid flange l5 and an annular disc 31 extending inwardly therefrom and mounted axially of the tube. The element 31 is, in this case, wholly supported by the grid elements, and the latter are swaged or brazed in the slots in the elements l5 and 31 respectively. While the actual disposition of the grid elements 35 is not a part of this invention, being described and claimed in a copending application of George Becker, they are preferably not truly radial but are either mounted on a slight angle with respect to the radii of the structure or are kinked adjacent one end so that both of their abutments do not lie on a single radius. As a result of this structure, differential expansion of the grid elements, due to heating from the electron stream. does not cause them to buckle but merely results in a slight relative rotation of the disc 31. This tilted grid bar structure is not necessary if soft copper grids are used and some other means is used to support the center portion of the grid cavity wall; such as using a few large bars bridging the same gap that the control grid bars are put across. These large bars would make the shadows needed for the large bars that might bridge the accelerator gap.

A grid-cathode tuning element is mounted within the filament column 21 and the flange 29. This element comprises a cylindrical cup 39, from the anode edge of which there projects an annular flange 4! which faces the grid support annulus 31. This arrangement is supported and positioned by a corrugated diaphragm 43 of this metal which is secured to the inner periphery of the frusto-conical flange 29. The bottom of the cup 39 is conical in form and its apex is secured to a thin rod or wire 45 extending substantially along the axis of the column 21 to a connection with a disc 41. The latter is supported from the foot II by metal bellows l3. Elasticity of the bellows holds the rod or wire 45 in tension, and by applying a greater or less amount of pressure against the disc 41 the relative position of the tuning flange 4| and the grid support 31 may be adjusted, changing the capacity between them and tunlng the cathode-grid cavity.

The cathode-grid cavity is excited by radiation from an antenna 5|, which projects axially into the cup 39 from the anode end of the device and is supported and excited as will later be described. Energy radiated from the antenna into the cup is transmitted radially outward into the cavity formed between the elements 43 and 31. The electrical dimensions of this cavity are adjusted by varying the position of the flange ll so that, at the frequency at which the device is to be operated, a potential node and current loop is formed at approximately the position of the tip of the arrow A, the exact position of the potential node varying slightly with the frequency to which the device is to be tuned. When so tuned the cavity between the end of the arrow A and the bottom or the groove 11 will also, of necessity, be onehalf wavelength in radial dimension at this frequency, with current loops at the two points mentioned. The cathode and grid lying midway, electrically, between these two points are positioned at a potential loop and therefore when the tube is excited there will be a maximum diiierence of radio frequency potential between them. Because of their close juxtaposition the characteristic resnatron grid-cathode action described in the prior Patent No, 2,424,002 above mentioned, takes place.

Turning now to the anode end of the device, the anode itself, in the sense of the collector of electrons from the cathode, is formed in the end of an annular flange 53 projecting inwardly of the device from the anode disc 3 on substantially the same radius as the center of the cathode 33. A V-section groove 55 is formed on the inner edge of the flange, the purpose of this groove being to extend the area over which the electrons from the cathode beam are collected and prevent excessive local heating, as has been described in the patent previously mentioned. The entire anode structure is preferably formed almost entirely oi massive copper in order to carry off the heat generated by the electron impact.

The accelerator-grid structure is carried on a column 51 which extends through a central aperture in the disc 3. The column is connected to the disc by means of a metal bellows 59 positioned between the outer surface of the disc and a collar 60 on the column. The column 51 terminates, at its inner end, in an annular flange 59 which faces the grid element 31. Spaced from this, toward the anode end of the tube is a second flange 6| for supporting the accelerator electrode proper. The flange 6| carries an extension 63 which is frusto-conical when viewed from the anode end of the tube and terminates in a lip or flange 65 projecting from the extension 63 in both directions, 1. e., both in the general direction of the cathode and of the anode. The ring BI connects to the anode flange 53 through a corrugated annular diaphragm G1, the latter being brazed to both elements.

Surrounding the lip or flange 65, and spaced therefrom so as to form a circular slot, is a similar but larger lip 39 forming the interior periphcry of a ring 1|, frusto-conical as viewed from the cathode end of the tube. The ring 2| termlnates, outwardly, in a flat annulus 53 which 7 carries, on the side toward the cathode, a pro- Jecting cylindrical skirt ll. The ring 13 is supported from the anode II by a corrugated annular diaphragm II, similar to the diaphragm I! but of larger diameter.

As was stated in the broad description of the invention above, the slot formed between the lips 85 and a should be substantially unobstructed. It has been found by experiment that the device is reasonably stable, mechanically, with no connections bridging this slot and it can be operated in this manner. A greater degree of mechanical rigidity can, however, be given to the structure by bridging the slot by a few conducting elements such as are indicated by the bridge 19, shown in the slot in the upper portion of the figure. Such bridges should be few in number, usually not more than three. In tubes of certain sizes, however, when operated on certain frequencies this number may be increased somewhat, say to six or eight or perhaps a very few more, in order to prevent the setting up of certain modes of circumferential oscillation which might otherwise occur. In general, however, the number should be kept down as far as is feasible since the bridges will intercept some of the electron flow from the cathode and therefore waste power. It will be seen that the elements 61, 63, 65, 88, ll, l3, l1, together with the anode itself, enclose an annular cavity with the anode groove 55 and the slot between the two lips located substantially at the median circumference thereof. Dimensions of this cavity are such that it is tuned approximately to the mean frequency of the range for which the tube is designed. Exact tuning within this range can be accomplished by moving the entire central column, flexing the metal bellows 59 and the diaphragms 61 and 11. When this is done the major tuning effect occurs by the change of capacity betweemthe tips of the anode itself and the flanges 65 and t9, the inductance around the cavity, at any radius, remaining substantially unchanged. Preliminary tuning, to bring the device within the desired range, can be accomplished by machining metal from the tips of the lips, thereby decreasing this capacity and raising the frequency, or by removing metal from the outer periphery of the cavity and thereby increasing the inductance and decreasing the frequency. The device is much more sensitive to the decrease in capacity than it is to the increased inductance for a given volume of metal and therefore great care must be taken, in this preliminary tuning operation, to avoid removing too much metal from the lips.

It will be understood that it is in the anode cavity that the useful high-frequency power of the device is liberated. Its energy is withdrawn by means of a tapered wave guide 8i which connects into the cavity closely adjacent the outer periphery of the anode 53 (l. e., closely adjacent to although not actually at a potential loop) it tapers outwardly, passing through the anode disc 3, its dimensions being such as to match the effective impedance of the anode cavity where it connects with it as well as the impedance of a wave guide to which it may be connected.

Considering this device mechanically there remains to be described the support of the input antenna i. The column 51 extends outwardly to beyond the flange 6i and terminates in a glass ring-seal 83. The latter in turn is sealed to an inwardly projecting skirt '5 from a disc 81. A central rod 89, carried by the disc 81, projects through the tubular column 51 and carries an ture in the grid element 31 into the cup 8!. In

operating the device the entire antenna structure is tuned so that a potential loop forms at the tip of the antenna; accordingly a node of potential is located approximately where it passes through the intervening elements in the active region of the tube.

In operation the device does not depart materially from that of resnatrons which have previously beendescribed. Excitation picked up from the antenna establishes a potential loop midway in the annular cathode cavity between the arrow A and the bottom of the groove II, with the result that the annular electron beam formed by emission from the cathode is densitymodulated as it passes through the grid. Electrons forming this beam are emitted in directions substantially normal to the cathode surface at the point of emission, and therefore they converge, considered along the radius of the tube, toward a focus which lies somewhere within the accelerator electrode slot. The grid structure is much closer to the cathode than this focus. The accelerator electrode is operated at the same D. C. potential as the anode, which may be as much as kv. positive to the cathode and the accelerating field is concentrated by the projecting lips 65 and 159 so that there is little defocusing effect from this field. The apposed faces of the flanges forming these lips are preferably not exactly parallel but are slanted so as to conform approximately to the electron paths which define the edges of the beam, so that the slot is only of sufllcient dimension to admit substantially all of the electrons constituting the beam.

In the cathode cavity the electron beam is subjected to a widely varying electrical field. This field does not, however, extend through the grid structure and the electrons are not affected by it after passing the grid. Since the grid elements conform to the same center of curvature as the cathode elements, the radial component of the accelerations imparted to the electrons due to grid potential are always nearly normal to the cathode surface. Considered circumferentially of the cathode, however, the field is not uniform and electron lenses are formed, at certain phases of the operation, which deflect the emitted electrons circumferentially to a greater or less extent. Because of the relatively large diameter of the cathode and of the slot in the accelerating electrode such deflections do not materially aifect the locations of the boundary of the electron beam; over the relatively short distances through which the electrons are deflected both the slot and the cathode may be considered as though they were linear rather than curved. The relatively large losses of power due to the interception of electrons by accelerator grid bars in devices of this character, even when those bars were placed nominally in the shadows of the control grid bars, are thereby avoided. Furthermore, owing to the very small dimensions of the control grid elements, consisting, as they do, of edge-on sheet material, a relatively much greater cathode surface can be employed. Where, for mechanical reasons, separate cathode elements are used (as distinguished from a. continuous unipotential indirectly heated cathode) the grid elements can be located on each side of each of the filaments and loss to the control grid can 9 be further minimized even though it is not entirely prevented.

The space between the grid and the accelerator electrode structure is, obviously, a cavity which might resonate. This cavity is, however, preferably so dimensioned that none of its readily excited modes of oscillation fall within the range over which the tube is desired to operate. Accordingly the oscillating potentials which are set up by such oscillations as do occur within this cavity do not accelerate or retard the electrons in the beam to an extent which cannot be tolerated, the only important potential existing in this region being the D. C. voltage between the grid and the accelerator structure. The electrons in the density-modulated beam therefore gather energy in their passage through the field between grid and accelerating electrode and neither part with or gain any considerable energy to high freouency fields (as distinct from the D. C. field) in this region.

After entering the anode cavity, however, since this is resonant and at uniform D. C. potential and since the electron beam passes through a potential loop in its transit across the cavity, the electrons are decelerated by the oscillating field only, their energy being expended largely in maintaining the oscillation within the cavity. Residuum of energy is, of course, expended as heat liberated at the anode surface. The efdciency of the tube depends very largely upon the extent to which the energy can be removed from the electron beam prior to the electrons entering the anode groove 55. It has already been indicated that the energy is drawn oil through the horn 8|.

The tube of Fig. l is designed primarily as a radio frequency amplifier. The energy fed into the tube to excite it may be derived from any source within the appropriate frequency range and such source may be coupled to the antenna through any of a number of well known devices.

The tube shown in Fig. 3 differs from that iilustrated in Figs. 1 and 2 primarily in that it is a radial-flow tube instead of one oi the axialfiow type. A secondary difference is that it is designed as a self-excited oscillator rather than as an amplifier but this is a matter of detail which does not affect, per se, the invention to which this specification is primarily directed. As in the tube first described it is shown with its cathode end at the left of the figure and its anode end at the right.

Like the device already described, the cathode and grid structures are supported on a cathode end disc IIII. This disc IOI carries, near its periphery, a skirt I03 to which is sealed a glass insulating and support ring III5 which connects the cathode end structure with the anode structure later to be described. The outermost column I Ill of the supporting structure projects inwardly toward the anode axially of the disc. This column carries a ring I 09 provided with a cylindrical flange III projecting toward the anode. An insulating seal II3 connects between the flange III and a similar flange II5 on a ring III supporting the grid structure, which is thus insulated from the cathode and so can be operated at a bias potential with respect thereto.

The cathode structure itself is in the form of a generally cylindrical cup, concentric with and fitting over the cathode support structure proper.

As in the previously described tube the cathode used in this embodiment of the invention comprises a multiplicity of directly heated filaments or requires two mutually insulated supports which act as the cathode leads. Column I01 forms one oi these supports and it also cooperates with the grid structure to form a highly attenuating coaxial transmission line which prevents any material escape oi power from the cathode-grid cavity. The cap comprising the grid structure includes an internally cylindrical portion I I! which is deep enough to enclose the column I ill for approximately one-half of its length. Within the cylindrical portion of the grid the column is enlarged in diameter by a number of annular mem bers. Taking the enlargements consecutively from the ring III! toward the anode, they comprise first a pair of annular cups I2I and I23, surrounding the column IllI with their open ends toward the anode. Each of these cups is approximately one-quarter wavelength in depth, considered at the mean frequency of operation of the tube. Together with the central column they form cavities generally L-shaped in radial section, which act as one-quarter wave chokes to waves of this frequency travelling outwardly from the tube toward disc IIiI. Continuing towards the end of the column it next bears an enlarged ring I25 of the same external diameter as the cups I2I and I23, and beyond this ring the column again projects at reduced diameter for a short distance to form the abutment I2I for the staple-shaped filaments I 23.

The second abutment for the filamentary cathodes is formed by a disc-like head I3I carried by a support column I33 within and spaced from the tubular column IIi'I. Column I33 is supported from the cathode-end disc I III by an external ring seal I35. Outwardly oi the tube, beyond the ring seal, column I33 carries a circular foot I34 from which the innermost column of the structure is supported as wil1 later be described.

The column I33 is generally tubular and, for most of its length, of uniform diameter. At the end adjacent the anode. however, the column is counterbored to a larger diameter, the counterbore merging with a disk-like cavity I39 formed in the end of the circular head I3I. This cavity is closed by a corrugated annular diaphragm I4I which supports the inner end of the grid-cavity tuning structure.

The grid-cavity tuner is generally similar in form and operation with that described in con nection with the tube of Fig. i. It comprises a deep cylindrical cup I43, the open end of which terminates in a fiat annular fiange I45. The bottom of this cup terminates in the innermost column I41, extending axially within the other columns already described and terminating in a disc I41. The latter disc is connected and sealed to the foot I34 of the column I33 by a metal bellows I49. A plurality of adjusting screws I5I journaled in the disc I41 and threaded into the foot I34 permit the tuner structure to be moved slightiy in an axial direction in order to accomplish its purpose.

The grid cap covers and surrounds the inner end of the entire cathode support and tuning structure. The major portion 01' this cap is truly cylindrical and of uniform diameter. The exception to this is in the portion forming the grid itself. The filaments I29, like those in the first described tube, are concave in the direction in which electron emission is desired and the portion of the grid cap immediately surrounding the filaments is bulged inwardly so that the grid bars I53, which are formed by slotting the grid cap, have a curvature about substantially the 11 same center as the concavity of the filaments themselves.

The inner diameter of the grid cap is sharply reduced in the plane of the inner end of the head iii. to form a cavity III of substantially rectangular radial section within which the cathode elements lie. Beyond this cavity the end of the cap is dished to form, with the diaphragm ill. an inner annular cavity I51 enclosing the tuning flange I. The grid cap is provided with central aperture II! of substantially the e diameter as the inside of the cup I" andHan exciting antenna I Bi projects through the aperture I59 and into the cup.

The anode-accelerator cavity is formed in a massive copper ring I81 which surrounds the grid and cathode in the plane of the radially directed electron beam. A deep V-shaped groove extending radially outward collects the electrons from the beam. The main body of the cavity I61 is generally lozenge-shaped in radial cross section and the slot through which the electrons enter is formed between flanged lips I88, spaced apart only sufficiently to admit substantially all of the electrons of the beam and with their apposed surfaces generally conforming to the electron paths defining the edges of the beam as in the previous case. Except for its diil'erent position the anode and accelerating electrode structure performs the same function in the same way as in the tube first described.

Extending axially outward from the anode ring I63 is a shallow but massive cup I'll enclosing the end of the grid cap. A coaxial transmission line enters the cavity between the cup and the grid cap centrally of the cup, terminating in a flat circular flange I15. The antenna IBI is mounted on the central conductor ill of this transmission line. The other end of .the line terminates in a small coupling loop I19. The line is of approximately the right length to feed energy picked up from the anode cavity back to the grid in the proper phase to sustain electrical oscillation of the tube, the exact electrical length of the line being adjustable by means of plugs iill which pass through the wall or the outer conductor I83 of the transmission and are sealed, vacuum tight, by means of the metal bellows I85.

Energy is withdrawn from the tube through the wave guide I81 which communicates with the anode cavity through a tapered matching section I89 and terminates in a horn antenna m.

Except in minor detail the operation of this tube is the same as that of the axial-flow tube first described. Energy is fed into the cathodegrid cavity from the antenna IBI, this cavity being tuned so as to form circumferential potential nodes and current loops at the position marked by the tips of the arrows B. For the frequencies at which the tube is designed to operate this also brings a current loop and potential node at appoximately the position indicated by the tip of the arrow C, thus placing the cathode in an annular cathode-grid cavity wherein a voltage loop is formed between cathode and grid. The position C may be considered the terminus of the attenuating coaxial line formed between the column lll'l and the grid cap, the axial length of the ring I25 being very nearly one-quarter wavelength for any of the operating frequencies and the section of the line formed by the ring terminating fli high l2 impedance formed by the choke I so that its impedance at its input end approaches zero. The conditions within the cavity I are therefore such as to give maximum grid control and eifectiveiy isolate thi cavity and cause it to operate independently.

The shape of the outer surface of the grid cap where it is surrounded by the accelerating electrode structure is such as to conform roughly to the latter. leaving a gap oi. fairly constant width wherein the electrical field for accelerating the beam between the grid and accelerating electrodes is concentrated by the projecting lips I". The cavity between the anode and grid is. as in the former case, of such dimension as to be non-resonant at the operating frequencies. In addition, the annular pocket formed between the flange Ill and the bottom ill of the anode cup is or such depth as to form an approximately short circulted quarter wave section, and therefore a high impedance is developed within the cavity at this point which tends to prevent energy at any of the operating frequencies, which may be developed in the grid-accelerator gap, returning through the cavity to the antenna iii and causing possible dephasing of the antenna potential.

The forms taken by the flanges which form the lips of the accelerator slot may be varied to suit the conditions in the particular embodiment of the inventionin which they are employed. It is, of course, desirable to make the slot as narrow as possible without materially interfering with the electron flow. Once the electron beam enters the slot there are no lateral forces tending to deflect the electron paths except the mutual repulsion of the electrons themselves. The concavity oi the cathode-grid structure may be such as to cause a focal or crossover position to lie entirely within the anode cavity or the crossover may occur within the slot itself. In the first instance the flanges converge in the direction of electron flow throughout their entire width, as indicated in Fig. 1. If the crossover takes place between the lips of the slot the flanges first converge and then diverge in the direction of flow as is illustrated in Fig. 3. Both of these arrangements are a refinement and the invention is operative if the walls of the slot are parallel.

The invention having been fully set forth, what is claimed is as follows:

1. A resnatron type vacuum tube comprising circular cathode-support elements and grid support elements juxtaposed to form a resonant cathode cavity, an annular cathode structure disposed within said cavity to direct a beam of electrons thereacross, said cathode structure being concave in the direction of desired emission so as to produce an annular beam of electrons which converges toward the center of curvature of the concavity, an annular grid disposed in the wall of said cavity in the path oi said beam and closer to said cathode than the position of maximum convergence of said beam, an annular anode-accelerator structure forming a resonant anode cavity having a substantially unobstructed annular slot therein positioned to admit said annular beam of electrons into said anode cavity, said anode-accelerator structure being so spaced with respect to said cathode that the maximum convergence of said electron beam lies within said slot and the width of said slot being only suilicient to admit substantially all electrons of said beam.

2. A vacuum tube structure in accordance with Gl 1- wherein aid annular slot is formed be- 13 tween lips, the opposed surfaces whereof are sloped in the direction of electron flow therethrough to conform substantially to the slope of the paths of the electrons defining the limits oi said beam.

3. A vacuum tube structure in accordance with claim 1 wherein said annular slot is formed between lips comprising annular flanges projecting toward said cathode structure.

4. A vacuum tube structure in accordance 'with claim 1 wherein said annular slot is formed between lips comprising annular flanges projecting both into said anode cavity and outwardly therefrom.

5. A resnatron type vacuum tube comprising an annular cavity resonator, an annular cathode positioned within the cavity of said resonator substantially at a current node therein, an annular grid structure forming a portion of the wall of said cavity substantially at a current node thereof opposite said cathode, said cathode being adapted to direct a narrow annular beam of electrons across said cavity and through said grid structure, and an annular anode cavity resonator having a substantially unobstructed annular slot in the wall thereo! positioned to receive said electron beam, the width of said slot being only sufficient to admit substantially all 01 the electrons of said beam, the edges of said slot forming an accelerator electrode for increasing the velocity of said beam in its transit between said cavities.

6. A resnatron vacuum tube in accordance with 14 claim 5 wherein said slot is formed between flanges projecting substantially parallel to the direction of electron flow within said beam.

7. A resnatron type vacuum tube in accordance with claim 5 including a plurality of conducting members bridging said slot, the angular separation of said members being large in comparison with the spacing of the elements constituting said grid 7 8. A resnatron type vacuum tube in accordance with claim 5 wherein said anode cavity surrounds said first-mentioned cavity, the flow of electrons in said beam being radial.

9. A resnatron type vacuum tube in accordance with claim 5 wherein said cavities are axially spaced, said cathode and said slot being of substantially equal diameters and the flow of electrons in said beam being in a direction generally parallel to the common axis of said annular cavity resonators.

DAVID H. SLOAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,409,693 Okress Oct. 22, 1946 2,409,694 Laidig Oct. 22, 1946 2,459,593 Sloan Jan. 18, 1949 2,489,298 Lafferty Nov. 29, 1949 2,506,752 Truell May 9, 1950 2,570,289 Touraton et al Oct. 9, 1951 

